X-Ray absorption in particular has been used as the basis for systems for scanning objects to create some form of representational image of the contents or components thereof. The thicker or more dense an object is then the more it will attenuate an x-ray beam. By use of suitable detectors and a suitable source, radiographs of an item under screening in the form of images based on the absorption of an object or set of objects can be generated.
Typically, an x-ray source generates an essentially two-dimensional beam and detectors of transmitted x-rays in one or two dimensional array are used to resolve transmitted information spatially into two dimensions based on transmitted x-rays (and hence differentiating by absorption). A computer is used to generate a two-dimensional image of the object from this spatially resolved information. In a refinement, it is known to build up successive two-dimensional image slices in cross-section and display these successively. Such a principle is employed in CAT scanning for example. Similar principles can be applied to imaging based on other interactions of source and object, for example based on backscattered radiation.
These known apparatus and methods tend to give limited information about the material content. In essence, at its simplest, all that is being measured is transmissivity of the object to the source radiation. Conventional detectors merely collect amplitude information discriminated spatially, but do not discriminate transmitted radiation spectroscopically.
However, it is known that spectroscopic information from transmitted x-rays could be used to give additional information about the material content of the objects or components being scanned. It is known that the x-ray absorption properties of any material can vary spectroscopically, and that the amount by which the absorption properties vary depends in particular on atomic number. Silicon-based dual band energy detectors have been used to generate pairs of images as low and high energy allowing some spectroscopic discrimination.
Recent development of detector materials that can resolve spectroscopic information about the transmitted X-rays more effectively has led to the development of apparatus that discriminate across a larger range of bands and generate a larger plurality of spectroscopically differentiated images. For example U.S. Pat. No. 5,943,388 describes a system that makes use of cadmium telluride detectors to image across at least three energy bands and generate at least three images. Such systems better exploit the effect of differential spectral absorption by different materials and enable a better approximation to be made between transmissivity and composition. However, the detector materials are expensive and difficult to fabricate, particularly if configured as a linear or area array with high pixel resolution.